In recent years, parallel with the traditional cellular networks for voice communication, there has been an increase in the development of widely-varying cellular networks for the mobile transmission of data. The significance of these networks may be perceived from the fact that some network service providers are already making inter-continental networks available.
These networks, however, present to some degree considerable differences in regard to their underlying technology, their fee structure, and the geographical area which they cover. Beyond this, the network subscriber or network user often requires specific interfaces for individual user programs.
Owing to the existing heterogeneous cellular infrastructure, and since the mobility of today's users frequently dictates that network subscribers go beyond the coverage area of one cellular network and move into the coverage area of another (i.e. the network subscribers "roam"), it is often necessary to operate with varying data-communication networks simultaneously.
Generic transmission systems which only allow specifically-defined communication with a single physical network are already known. In the simplest instance, this type of system consists of a highly-specific applications program interface (API); in the most complex case they provide a standard protocol for data transmission to the user. There is, however, no known system of this type which integrates the varying physical networks under a single standard protocol.